1. Field of the Invention
The present invention deals with an improved drive circuit for driving a magnetic bubble device such as magnetic bubble memory.
2. Description of the Prior Art
A well-known method of driving magnetic bubbles is, generally, based on the use of a rotating magnetic field and a permalloy pattern. According to this method, a T-bar shaped permalloy pattern, for example, is formed on the surface of a crystal which allows the existence of magnetic bubbles by way of metal vaporization or etching, and a rotating magnetic field is applied from an external unit. The rotating magnetic field can usually be obtained by flowing electric currents having phases different by 90.degree. from each other to a coil X and a coil Y that are disposed in a crossing manner. The magnetic bubbles are propagated along the permalloy pattern in accordance with the rotating magnetic field. The circuits for driving such a magnetic bubble device by means of a rotating magnetic field can be roughly grouped into a sine wave drive circuit and a triangular wave drive circuit (U.S. Pat. No. 3,952,292) featuring various respective merits and demerits. For instance, the triangular wave drive circuit has the advantages that it does not require a resonant capacitor such as is needed by the sine wave drive circuit, and that it does not require resonant adjustment. The triangular wave drive circuit, however, has certain disadvantages in that it consumes increased electric power during high frequency operations as compared with the sine wave drive circuit and that it is virtually not usable at rotating magnetic field frequencies of higher than 200 KHz. Because of these reasons, the triangular wave drive circuit is used for small-size and small-capacity magnetic bubble memories where the circuit is used in the form of a one-chip package and at low speeds to feature its advantages.
In the conventional triangular wave drive circuit, however, unless the time for rendering the control switches conductive to cause currents to flow which increase toward the positive direction to the drive coils X and Y, is set to be strictly equal to the time for rendering the control switches to cause currents to flow which increase toward the negative direction, the d-c level of the drive current is shifted toward either the positive direction or the negative direction. This means that a direct current is added to the triangular drive current, causing the vector of the rotating magnetic field to undergo parallel translation toward a given direction, presenting a serious problem in the operation of magnetic bubbles.